The present invention relates to servo means for controlling the orientation of automobile headlamps as a function of the steering mechanism position, notably in the case of headlamps adapted to pivot on either side of a middle position.
It is known, with a view to obtain a better front visibility along sinuous or winding road sections, to control the direction of the light beams of turning front headlamps of a motor vehicle in response to the actuation of the steering mechanism or to the position of the steerable wheels, the headlamps pivoting laterally in relation to the direction of the path followed by the vehicle.
Hitherto known control devices of this type are based on the use of more or less complicated linkage systems, of flexible cables or rods driven through mechanical, electromagnetic or hydraulic means providing not only the lateral scanning function but also necessary ancillary correcting functions, notably the function of returning the device to the "straightahead" position requiring a considerable precision and reliability in time. These requirements, notably in the case of hydraulic systems, are frequently impaired by expansion effects due to thermal variations causing the volume of the operating fluid to increase or decrease. These variations are in turn a cause of undesired additional or insufficient movements leading to a certain detrimental misadjustment of the desired angular position of the headlamps. Obviously, although these disturbances are scarcely discernible or objectionable during angular scanning manoeuvers, they are detrimental and sometimes hazardous on account of their permanent nature when the vehicle is driven on straight roads.
Various devices have already been proposed for preventing these disturbances, notably by associating with the main headlamp hydraulic control circuit an auxiliary circuit wherein any volume variations are utilized for compensating variations due to the temperature prevailing in the main circuit, such auxiliary circuit being controlled by the expansion of an element heated by an electric circuit energized through means responsive to the movements of the headlamp proper. This system is objectionable because on the one hand it introduces a heating member acting with a certain lag or response time, and on the other hand it adds to the hydraulic system an electric circuit and a switch for supplying current to the heating element, with a consequent likelihood of impairing the reliability of the control system.